Liquid crystal displays (LCDs) are widely used in computer monitors, laptop computers, tablet computers, cellular telephones, video players, cameras, watches and television sets, as well as other devices. LCDs are chosen because they are small in size, light in weight, low in power consumption and able to produce high-resolution images. LCDs are often in the form of display panels having row and column address lines coupled to pattern generator electronics which generate patterns or video on the display. As is well known, the voltage required by the LCD is often much greater than the voltage handling capacity of the pattern generator electronics. Therefore, it is common to utilize a level shifter circuit between the pattern generator electronics and the display to convert a lower voltage signals from the pattern generator electronics to the voltage required to drive the display.
The row lines of the display can be modeled as a capacitor in series with a resistor. As is well known, the power required to charge up the capacitive load is a significant portion of the power required to drive the display. Thus, it is common to share this charge between a first row line and a second row line, which are operated out of phase with each other. This charge sharing phase is illustrated in FIG. 1 generally as 100. The two clock signals 102, 104 are 180° out of phase with each other and share charge during the time periods shown in the rectangles 106, 108 and 110. In rectangle 106 charging can be seen at 112 and the discharging can be seen at 114. In rectangle 108, the discharging can be seen at 116 and charging can be seen at 118. In rectangle 110 charging can be seen at 120 and discharging can be seen at 122
A prior art LCD system having an LCD panel 204 and a driving circuit 202 is shown in FIG. 2 generally as 200. The driving circuit 202 includes circuitry for performing the charge sharing shown in FIG. 1. In FIG. 2, each of the row lines of the LCD panel 204 is modeled as a resistor RASG in series with a capacitor CASG. Circuit 202 comprises, inter alia, a gate driver circuit 218 driving buffers 206, 208 and providing a signal to charge transfer transistors 214, 216. Diodes 210, 212 prevent reverse current flow. When activated, transistor 214 allows current to flow as shown by dotted lines 220, and transistor 216, when activated, allows current to flow as shown by dotted lines 222. Resistors RCS and our RBCS control the slope of the charge/discharge in order to reduce visual effects on the LCD panel. One disadvantage of the circuit is that it requires two pins or balls for each channel.
FIG. 3 shows a prior art circuit 300 which utilizes transistors 314, 316 in place of the diodes 210, 212 shown in FIG. 2, and which allows the utilization of a single resistor R1 in place of the two resistors RCS and RBCS shown in the circuit of FIG. 2.
FIG. 4 shows a prior art circuit for driving four row lines or channels generally as 400. For ease of illustration, the integrated circuit 402 is shown divided into a left side and right side circuit, but those skilled in the art will recognize that only a single integrated circuit is required. This circuit utilizes the single resistor charge sharing scheme shown in FIG. 3. The row lines 406, 414, 422, 424 are each modeled as a resistor in series with a capacitor to ground. The driver circuit for row 406 comprises a push-pull circuit consisting of transistors M1a and M2a coupled between VDD and VSS and having an output coupled to channel CLK1. Transistors M3a correspond to the transistors 314, 316 shown in FIG. 3, which replace the diodes 210, 212 in FIG. 2. Parasitic capacitor 404 is present at pin CLK1 and parasitic capacitor 408 is present at pin CS1. These parasitic capacitors are a result of the electrostatic discharge (ESD) circuits at those pins as well as the parasitic capacitances of the transistors M3a. 
Similarly, the driver circuit for row 422 comprises a push-pull circuit consisting of transistors M1b and M2b coupled between VDD and VSS and having an output coupled to channel CLKB1. Parasitic capacitance 410 is present on pin CLKB1. The driver circuit for row 414 comprises a push-pull circuit consisting of transistors M1c and M2c coupled between VDD and VSS and having an output coupled to channel CLK2. Transistors M3c correspond to the transistors 314, 316 shown in FIG. 3, which replace the diodes 210, 212 in FIG. 2. Parasitic capacitor 412 is present at pin CLK2 and parasitic capacitor 416 is present at pin CS2. These parasitic capacitors are as a result of the ESD circuits at those pins as well as the parasitic capacitances of the transistors M3c. The driver circuit for row 424 comprises a push-pull circuit consisting of transistors M1d and M2d coupled between VDD and VSS and having an output coupled to channel CLKB2. Parasitic capacitor 420 is present at pin CLKB2.
Although this circuit allows the resistors RCS to be shared between a channel pair [RS1] and thus reduces the pin count, it creates a new problem for the latest LCDs and fails to solve an older problem. A problem is created for the latest LCDs in that driving channel 406, for example, includes the effects of the parasitic capacitance 408, whereas driving line 422 does not include this parasitic capacitance. A similar situation exists when sharing charge between the two row lines. Thus, the slope of the charging (rising) and discharging (falling) waveforms will be different, as illustrated in FIG. 5A showing the slope of the charging waveform and FIG. 5B showing the slope of the discharging waveform. This difference in slope creates a DC offset as shown in FIG. 6, generally as 600. The DC offset is shown at the right side of the waveform. This can create objectionable visual effects on the latest LCDs. Furthermore, the older problem that is not solved, is the fact that although resistors are shared between two channels, different resistors, normally discrete components, are utilized for each pair of channels. There will be a difference in resistance between these shared resistors, which can give rise to differences in the slope during charge sharing, which may result in objectionable visual effects on the display.
Thus, there is a need for improved level shifter for LCD applications.